How To Design a Singlet Lens

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http://www.zemax.com/kb/articles/20/1/How-To-Design-a-Singlet-Lens/...
ZEMAX Users' Knowledge Base - http://www.zemax.com/kb
How To Design a Singlet Lens
http://www.zemax.com/kb/articles/20/1/How-To-Design-a-Singlet-Lens/Page1.html
By Dan Hill
Published on 21 July 2005
This article explains:
Modifying system settings, including System Aperture, Lens Units, Fields and Wavelengths
Entering lens prescription data
Using solves to enforce design contraints
Analyzing system performance prior to optimization
Determining degrees of freedom and setting variables
Setting up a default merit function
Optimizing and analyzing final design performance
Introduction, Lens Prescription and Design Constraints
The singlet (a single lens) is arguably the simplest imaging system modeled in ZEMAX.
Nevertheless, the design of this simple imaging system can help introduce you to the interface of
ZEMAX, touch on fundamental design concepts and strategies, and demonstrate how to use some of
the basic analysis features for optimizing and determining optical performance.
In this particular exercise, we will design and optimize an F/4 singlet lens made of N-BK7 glass. The
final design solution shall meet the following specifications and constraints:
Focal Length = 100mm
Semi-Field-Of-View (SFOV) = 5 degrees
Wavelength: 632.8nm (HeNe)
Center Thickness (c.t.) of the singlet: 2mm < c.t. < 12mm
Edge Thickness (e.t.) of the singlet: e.t. > 2mm
The singlet shall be optimized for smallest RMS Spot Size averaged over the field of view at the
given wavelength
Object is at infinity
Given ZEMAX's user-interface and available tools, the singlet can be modeled and optimized easily!
The Lens Data Editor
In computer-aided sequential lens design, rays are traced from one surface to the next in the order in
which they are listed. To do this, ZEMAX uses a spreadsheet format called the Lens Data Editor
(LDE).
Upon opening ZEMAX, a blank LDE will appear within the main ZEMAX window. The main ZEMAX
window has a large blank area, with a title bar, menu bar, and toolbar at the top. The LDE is the
primary spreadsheet where the majority of the lens data is entered. Some of the main entries include
the following
Surf: Type
the type of surface (Standard, Even Asphere, Diffraction Grating, etc)
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Comment
an optional field for typing in surface specific comments
Radius
surface radius of curvature (the inverse of curvature) in lens units
Thickness
the thickness in lens units separating the vertex of the current surface to
the vertex of the following surface
Glass
the material type (glass, air, etc.) which separates the current surface and
the next surface listed in the LDE
Semi-Diameter
the half-size of the surface in lens units
Each row within the LDE represents a single surface. In sequential ZEMAX, each optical system
begins at the object (OBJ) and ends at the image (IMA). In addition to the object and image planes,
one of the surfaces must be defined as the aperture stop (STO).
Data can be entered into the LDE by typing in the required values in the highlighted cell. The cursor
keys or the mouse may move the highlighted bar to whichever column is desired.
Defining System Settings
Most frequently, the system aperture is the first parameter which is defined when starting a new
design. The system aperture not only defines the size of the beam which ZEMAX will trace through
the optical system, but it also determines the initial direction cosines of the rays launched from each
field point in the OBJ plane. The system aperture can be defined by a number of different types,
including Entrance Pupil Diameter (EPD), Image Space F/#, Object Space NA, Float By Stop Size,
etc. Each of these types are defined in more detail in the following section of the ZEMAX User's
Guide: “Chapter 6: System Menu > General > Aperture.”
Entrance Pupil Diameter is perhaps the most commonly used system aperture type and is the most
convenient definition for the current example. In ZEMAX, the EPD is defined as the diameter of the
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pupil in lens units as seen from object space.
We can easily determine the EPD required for the singlet lens. As was outlined earlier, the singlet
lens must have an F/# equal to 4 and an effective focal length of 100mm. Since the F/# is the ratio of
the paraxial effective focal length at infinite conjugates over the paraxial entrance pupil diameter, the
appropriate EPD is 25mm:
Where is this value entered into ZEMAX? The system aperture, as well as other system specific
settings, are controlled by the System General dialog. To access the System General dialog, select
System > General from the main menu in ZEMAX, or click the “Gen” button on the toolbar (often
referred to as the “button” bar).
Once the System General dialog is opened, we can now enter in the appropriate system aperture
type and value for current design. Under the Aperture tab of the System General dialog, select
Entrtance Pupil Diameter as the "Aperture Type", and enter in a value of 25 for the "Aperture Value."
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The Aperture Value is considered to be in Lens Units. The lens units define the units of measure for
dimensions in most of the spreadsheet editors in ZEMAX. These dimensions apply to data such as
radii, thicknesses, EPDs, and most other parameters in ZEMAX. It is very important that the system
units be defined prior to starting the design. Always check to verify that the system lens units are
what you expect them to be!
There are four choices for lens units in ZEMAX: millimeters, centimeters, inches, or meters. For the
purposes of this design, millimeters will be used. Under the Units tab of the System General dialog,
select Millimeters as the "Lens Units."
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Click "OK" to close the System General dialog. For the time being, other system settings can be
ignored and left as the defaults.
Defining Fields in ZEMAX
Field points from within ZEMAX are defined in the Field Data dialog. To access the Field Data
Dialog, select System > Fields from the main menu, or click on the “Fie” button on the button bar.
ZEMAX supports 4 different models for defining fields:
Angle (Deg)
the angle in degrees that the chief ray makes with respect to the
object space Z axis. Note that by definition, the chief ray passes
through the center of the entrance pupil, so the field angles are
measured with respect to the center of the entrance pupil. Positive
field angles imply positive slope for the ray in the direction of
propagation, and thus refer to negative object coordinates. This
option is most useful when at infinite conjugates.
Object Height
the X and Y heights directly on the location of the object (OBJ)
surface. The heights are measured in lens units. This option
cannot be used when at infinite conjugates.
Paraxial Image
Height
the paraxial image height location on the image surface. This option
is useful for fixed-frame size designs, such as photographic film in
camera systems. This option only works well with systems which
are well described by paraxial optics.
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Real Image Height
the REAL image height on the image surface. This option is also
useful for fixed frame designs. However, ray tracing with this option
is slightly slower since ZEMAX must use an iterative approach to
determine the proper real ray coordinates of the chief ray on the IMA
plane.
For the purposes of the singlet design, we will define the fields in terms of angle. Rather than having
a single field representing the HFOV, three fields will be defined within the requirement of 5 degrees:
(0, 0), (0, 3.5), and (0, 5).
Currently, 12 fields can be entered into the Field Data dialog. Each of these fields can be given a
weight, which is primarily useful in optimization. However, for the purposes of this design, all field
weights will be left at 1. Enter the three fields into the first three entries in the Field Data dialog, as is
shown below:
Select "OK" to close the Field Data dialog.
Setting the Wavelengths
Wavelength data is entered into ZEMAX much like field data, only the wavelengths are entered into
the Wavelength Data dialog. You may access the Wavelength Data dialog by selecting System >
Wavelengths from the main menu, or by pressing the “Wav” button on the button bar.
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This singlet design is purely monochromatic (pertaining to a single wavelength). From the inital
design specifications, the wavelength which will be used is 0.6328mm...the wavelength of a HeNe
(Helium Neon) laser.
This wavelength may be manually typed into the Wavelength Data dialog, or it may be entered by
selecting one of the pre-programmed wavelength options in the pull down menu near the bottom of
the Wavelength Data dialog:
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F, d, C (Visible) is the first option by default. However, choose the current design wavelength by first
selecting HeNe (.6328) from the pull-down menu, and then pressing the “Select ->” button which
is next to the pull-down menu. ZEMAX will automatically place this wavelength into the first entry:
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Note that wavelengths in ZEMAX are always entered in microns, regardless of the system lens units!
Click "OK" to close the Wavelength Data dialog and apply the defined wavelengths.
Note that wavelength weights are also supported, and are used for various purposes within ZEMAX.
However, for the scope of this design, all wavelength weights will remain unity.
Inserting Surfaces
Once the system settings have been defined, information specific to each surface can be entered into
the Lens Data Editor. To reiterate, each row in the LDE represents a single surface. Therefore, two
surfaces separated by glass comprise a single element. So, for the purposes of the singlet, a total of
4 surfaces are needed:
1. The object surface
(OBJ):
the location where rays are launched
2. The front surface of the
lens:
where the rays enter the lens. For this design, this will also be
the lcation of the stop (STO).
3. The back surface of the
lens:
where the rays exit the lens back into air
4. The image surface
(IMA):
the location where the ray trace stops. No surfaces can be
located after the image surface in the LDE. Note that this surface
does not necessarily have to be at an acutal image location.
By default, only three surfaces are included in the LDE. Surfaces may be added to the LDE using
the “Insert” key on the keyboard, or by selecting Edit > Insert Surface on the menu bar of the LDE.
When using this method, a surface will be added prior to the row in which the highlighted cursor is
currently located. To add another surface after the current surface, “Ctrl + Insert” or Edit > Insert
After may be used.
Since the stop will be located at the front surface of the singlet lens, insert another surface
(representing the back face of the lens) by placing the highlighted cursor in the row of the IMA
surface, and pressing the “Insert” key:
The Comment column in the LDE can be very helpful in keeping track of what each surface
represents. To enter a comment for a surface, highlight the appropriate cell, and type in the desired
text. Once finished, hit the “Enter” key or move the cursor to another cell by using the arrow keys.
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Entering comments as you move along is a very good habit to get into. For the singlet, identify each
surface by typing the following text into each appropriate cell in the LDE.
Entering Lens Data
The singlet will be made of N-BK7 glass. In ZEMAX, this is the material separating the front and
back surfaces of the lens. Enter the glass type separating these two surfaces by simply typing the
name of the glass (N-BK7 in this example) in the appropriate cell in the LDE.
ZEMAX automatically recognizes this glass type as one of the many glasses which are compiled into
the built-in Glass Catalog. The Glass Catalog contains all of the necessary information for hundreds
of glasses provided by manufacturers around the world. ZEMAX will automatically look up this glass
in its database to determine the index of refraction of the material at each design wavelength.
Once the glass type is entered into the LDE, the lens thickness for the singlet may be typed into the
Thickness column for Surface 1. Since the thickness is the distance along the optical axis to the next
surface, this becomes the center thickness of the lens element. As a starting point, a thickness of
4mm may be applied as it is a reasonable center thickness for an aperture of 25mm. Type in a value
of 4 into the Thickness column for Surface 1. Note that this parameter will later be set as a variable
for optimization.
Similarly, the radius of the first surface and the thickness between the back of the lens and the image
need not be pre-determined since they will be set as a variables for optimization. For the time being,
we will leave the Radius of Surface 1 as Infinity and change the Thickness of Surface 2 to 100mm.
Type in the value of 100 into the Thickness column of Surface 2.
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Solves
When given constraints on an optical design, there are two possible methods of upholding these
constraints:
1. Make the parameters which affect these constraints variables and add boundary constraints
into the Merit Function Editor (to be introduced shortly) OR
2. Use special solves to enforce the constraints, eliminating unnecessary variables.
The latter of these two is far superior. Though both provide a method of adjusting lens parameters to
maintain a specified constraint, boundary constraints can slow down execution of the merit function.
There are many different solves available within ZEMAX, each of which has a specific purpose.
However, the performance specifications for this design calls for the use of only one of these solves:
one to set the system F/# to maintain the desired focal length.
To activate a solve dialog, right-mouse-click on the desired cell, or highlight the cell and press “Enter”
on the keyboard. Depending upon which parameter the solve is activated, different solves are
available.
To maintain the system F/#, an F Number solve can be placed on the radius of Surface 2. The F
Number solve adjusts the final optical surface curvature to maintain the system focal length. Rightmouse-click on the Radius cell for Surface 2 to activate the Curvature solve on surface 2 dialog.
Select F Number from the "Solve Type" pull-down menu and type in a value of 4 into the "F/#" entry
below.
Click "OK" to close the solve dialog.
Once the F Number solve is set, ZEMAX will automatically adjust the radius to maintain the desired
F/#. In other words, anytime a lens parameter is altered, the solve will be automatically
re-calculated. The letter "F" next to the radius is indicative of the F Number solve in place.
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Evaluating System Performance Prior to Optimization
There are many different analysis features included in ZEMAX, each of which can be used
to evaluate the performance of a design. In this exercise, we will use four of the more fundamental,
commonly known types of analyses of system performance to evaluate the singlet prior to
optimization:
Layout
A layout may be opened by selecting a Analysis > Layout > 2D Layout from the
main menu, or by pressing the “Lay” button on the button bar. The 2D Layout
option plots a YZ cross section through the lens, and is only valid for rotationally
symmetric, axial systems. A layout diagram of the current system is always a
useful visual representation of the current optical system.
Spot
Diagram
A spot diagram may be accessed by selecting “Analysis > Spot Diagrams >
Standard” from the main menu, or by pressing the “Spt” button on the button
bar. The spot diagram gives indication of the image of a point object. In the
absence of aberrations, a point object will converge to a perfect image point. By
default, ZEMAX plots the spot diagram for each field point.
OPD Fan
The Optical Path Difference (OPD) fan can be opened by selecting “Analysis >
Fans > Optical Path,” or by pressing the “Opd” button on the button bar. The
OPD fan is a plot of the optical path difference as a function of pupil coordinate.
In a perfect optical system, the optical path of the wavefront will be identical to
that of an aberration-free spherical wavefront in the exit pupil.
Ray Fan
The Ray Fan plot in ZEMAX may be opened by selecting “Analysis > Fans >
Ray Aberration” from the main menu in ZEMAX, or by selecting the “Ray” button
on the button bar. The Ray Fan plots ray aberrations as a function of pupil
coordinate. Generally, a given ray which passes through the optical system an
onto the image surface, its point of intersection falls on some small but nonzero
distance away from the chief ray. Once again, in a perfect optical system, the
ray aberrations should be zero across the pupil.
Note that the Spot Diagram, OPD Fan, and Ray Fan Plot are some of the most important tools that a
lens designer has available for determining the different types and magnitudes of aberrations present
in an optical system. However, the process by which a designer can determine the aberrations
present in his/her deisgn from these analysis features goes beyond the scope of this exercise.
Instead, these concepts are covered in great detail in several of the references mentioned in Chapter
1 of the ZEMAX User’s Guide.
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Given the arrangement of the design we have constructed so far, open each one of the above
analysis features to review the current lens performance.
From evaluation of the four plots above, it is obvious that the singlet design has a significant number
of aberrations, including but not limited to spherical, coma, distortion, defocus, field curvature, and
astigmatism. In addition, the geometrical and RMS radii (as is reported at the bottom of the Spot
Diagram) at the maximum field are roughly 734.581 and 1774.42µm, respectively.
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Using the Quick Focus Tool
As could be seen from the four analysis features, the performance of the singlet at this point is
certianly not optimal. A big factor in this fairly poor performance was the random selection of the
location of the image plane. Even from the Layout, it is obvious that the currently selected image
plane is not at "best focus."
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Even prior to optimization, we can use a tool within ZEMAX to better position the current image plane
location. The tool is known as the Quick Focus tool. Quick Focus is a feature in ZEMAX which
adjusts the thickness of the surface prior to the image plane to minimize the RMS aberrations.
Open the Quick Focus dialog by selecting Tools > Miscellaneous > Quick Focus or by pressing
"Shift+Ctrl+Q" on the keyboard. The "best focus" position which this tool will choose depends upon
the criterion which is selected. For the singlet design, we will use radial spot size with respect to the
centroid. Select Spot Size Radial and place a check next to the "Use Centroid" box.
Click "OK" to close the Quick Focus tool dialog.
Note the automatic change to the thickness of the surface prior to the image plane. Update each of
the opened analysis windows by selecting "Update" in the upper-left hand corner of each individual
analysis window. Note the fairly significant changes in performance simply due to the new selection
of the image plane location. Most importantly, the RMS and Geometrical spot sizes at the maximum
field decreased by nearly a factor of 2!
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Even though the design is undoubtedly better than before, there is still room for improvement.
Setting Variables and Constructing the Default Merit Function
There are certainly limits as to how well a singlet can perform, but ZEMAX can still be used to find a
better solution than the one which currently exists. In doing so, it is important to first determine how
many degrees of freedom the current design has. That is, how many parameters are free to adjust?
For the singlet in this exercise, one of the parameters (the Radius on Surface 2), can no longer be
considered a freely varying parameter since it is controlled by a solve to meet a specific design
constraint. However, the center thickness of the lens (the Thickness on Surface 1), the radius of
curvature of the front surface (Radius on Surface 1), and the distance from the back of the lens to the
image plane (the Thickness on Surface 2) can all be varied in attempt to minimize the RMS spot
radius of the singlet.
To allow ZEMAX to consider a parameter as a degree of freedom during optimization, a Variable
solve type must be placed on the cell in the LDE which represents that parameter. You may set the
solve type by right-mouse-clicking on the desired cell or by highlighting the appropriate cell and
pressing Ctrl+Z on the keyboard. In the solve dialog which appears, select Variable as the "Solve
Type" and press "OK." The letter “V” next to the parameter is indicative of a variable in place. Place
a variable solve on all three of the parameters which are free to vary during optimization:
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Once the variables are set, we can now construct the Default Merit Function. The merit function is
constructed in a completely separate editor from the LDE, called the Merit Function Editor (MFE).
Open the MFE by selecting Editors > Merit Function from the main menu in ZEMAX.
The merit function is a numerical representation of how closely an optical system meets a specified
set of goals. From within the MFE, ZEMAX uses a list of operands which individually represent
different constraints or goals for the system. Once the Merit Function is complete, the optimization
algorithm in ZEMAX will attempt to make the value of the merit function as small as possible.
Although it is possible to construct a merit function by hand, it is much easier to have ZEMAX
construct one for you. A default merit function can be constructed by selecting Tools > Default Merit
Function from the menu bar in the MFE.
Upon selecting this option, the Default Merit Function dialog will appear, from which various options
may be selected for defining the default merit function. Each of the options available in this dialog is
discussed in detail in Chapter 14 of the ZEMAX User’s Guide.
For the current exercise, the singlet is to be optimized for RMS Spot Radius with respect to the
centroid, all of which are options already built-in to ZEMAX’s default merit function capabilities.
Select RMS, Spot Radius, and Centroid under the Optimization Function and Reference portion of
the Default Merit Function dialog.
To prevent the singlet from becoming too thick or to thin, it is important that we set boundary
constraints on the thickness of this element. The default merit function has options to set boundary
constraints on both glass and air thicknesses. By checking the “Glass” option, minimum, maximum,
and edge thickness values can be manually typed into the appropriate entries.
As was described in the system requirements, the singlet center thickness shall be no larger than
12mm, no smaller than 2mm, and shall have an edge thickness greater than 2mm. Type the
appropriate values into the Default Merit Function dialog for the "Min", "Max", and "Edge" glass
thickness entries.
Other than the selection of the appropriate Optimization Function and Reference and the Thickness
Boundary Values modification, all other parameters may be left as the default for the purposes of this
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exercise.
Click "OK" to close the dialog.
Performing Optimization
After selecting “OK” in the Default Merit Function dialog, note the operands which are automatically
inserted into MFE. Each of these operands have a particular Target, Weight, and Value which
contributes to the value of the merit function, located in the upper left hand corner of the MFE.
During optimization, ZEMAX attempts to lower this merit function value, which means the design is
closer to the goal described in the Merit Function Editor.
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To perform Optimization, select “Tools > Optimization > Optimization from the main menu, or press
on the “Opt” button located on the button bar. Either of these actions will open the Optimization
dialog box. Note that within the Optimization dialog, there are a number of different cycles to choose
from. Selecting “Automatic” will ask ZEMAX to run the optimization routine until it has found a local
minimum, a solution to the current merit function.
Note that ZEMAX reports both the Initial MF as well as the Current MF values. Run the optimization
by pressing the "Automatic" button, and note the change in the merit function value:
BEFORE
AFTER
Click "Exit" to close the Optimization dialog.
Evaluating Final System Results
Now the that optimization routine is complete, we can evaluate the final design performance and
ensure that all of the initial design contraints are met. Each of the previously opened analysis
windows can be updated by selecting "Update" from the menu bar of each individual graphics
window.
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Ultimately, ZEMAX has optimized a singlet lens under the constraints which were given in the initial
system requirements. Compared to the initial performance analysis, the RMS and Geometrical Spot
radii have dropped by nearly a factor of 10! It is also important to note that ZEMAX's chosen
thickness for the lens falls within the desired range, and the edge thickness is certainly greater than
2mm; each of which meet the initial system requirements.
Though the performance of the singlet not diffraction limited, the process by which the final design
was achieved can be applied to much more complex, more-desirable optical systems!
Summary and References
This tutorial has outlined the basic process of designing a lens, analyzing its performance, and
optimizing under certain design contraints. In summary:
Given certain design criteria, it is important to begin the design by entering in the appropriate
system settings, such as Aperture Type, Wavelengths, Fields, and Lens Units.
Once system related settings are complete, the lens prescription data can be entered into the
Lens Data Editor. Educated guesses may be made to those unkown parameters to define a
starting point for optimization.
Use solves to enforce design contraints if at all possible.
Prior to optimization, the system performance may be analyzed by any number of availalbe
features in ZEMAX.
Determine the degrees of freedom and select the appropriate variables for optimization.
You may use the built in tools to create a default merit function. Remember, the operands
within the merit function define the goal which you are attempting to achieve by optimization.
The lower the merit function value, the closer you are to your target.
Once optimization is complete, ensure that the final design measures up to the inital contraints
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and requirements. Otherwise, more optimization techniques may be required.
References
There were no external references used for this article.
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How To Design a Singlet Lens

Transcription

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